378 alcoholic phenolphthalein solution. The number of c.c. of N/io-alkali required, increased by one-tenth, represents the volatile acid number. For each series of determinations a blank experiment must be made with the same glycerine (20 grams), sodium hydroxide solution (2 c.c.) and sulphuric acid (50 c.c.) and under the same conditions as in the actual test. The number of c.c. of N/io-alkali used in this blank experiment is subtracted from the volume used in the actual test. The determination of the volatile acid number may be carried out along with that of the Polenske number (see Butter, 15, in Chapter II of Vol. II). The Italian official method for determining the volatile acid number is, for oils, Wollny's modification of the Reichert-Meissl method, and for butter, Leffmann and Beam's modification, which differs from that described above in using a glycerine and soda solution previously prepared (125 c.c. of glycerine and 25 c.c. of 5% sodium hydroxide solution heated until the water is eliminated) in place of glycerine and soda separately, and in a few other detail's. The volatile acid number is of importance in the analysis of only a limited number of fats, principally butter. With most oils and fats, the number is less than i ; coconut oil, palm-kernel oil, croton oil, cacao butter and a few other fats have numbers above i (up to 14), while for butter the value is 28. Some fish oils and other marine animal oils (dolphin, whale) have variable and sometimes moderately high volatile acid numbers. 11. Acetyl Number This represents the number of milligrams of potassium hydroxide corre- sponding with the quantity of acetyl (CaHgO) combining with i gram of fat or wax, or, more usually, with i gram of fatty acids or higher alcohols (unsaponifiable substances) obtained from a fat or wax. This number is determined as follows : About 20 grams of the substance or of the free fatty acids obtained in the manner described under " Saponi- fication " (see 5, above) or of unsaponifiable substances (high alcohols, etc.) obtained as indicated under " Non-saponiftable Substances " (see ig, below) are boiled for 2 hours with an equal volume, of acetic anhydride in a flask fitted with a reflux condenser, the mixture being subsequently transferred to a beaker, mixed with 500 c.c. of hot water and boiled for half an hour. The supernatant water is then siphoned off and the residue again washed in the same way, this treatment being continued until the water no longer becomes acid ; this usually requires four or five washings. The acetylated product is then filtered through a dry paper in an oven at 100° and used for the determination of the acidity number—known as the ccdyl acid value —and the saponification number—the acetyl saponification number. The acetyl number is given by the difference between these two. For these determinations, 3-5 grams of the acetylated product are dissolved in 50 c.c. of 90% alcohol and the solution titrated with seminormal potassium hydroxide solution in presence of phenolphthalein ; this gives the acetyl acid number. The same liquid is then boiled for a short time in a water-bath with excess of N/2-KOH and alcohol and the excess of alkali titrated with N/2-HC1, this giving the acetyl saponification number. EXAMPLE : 3-402 grams of acetylated fatty acids from castor oil required the results taken, provided t hat these do not differ greatly.quid remains clear.r.